Angiostatin is a protein that naturally occurs in animals, including humans. It works by blocking the growth of new blood vessels, which is important for cancer growth. Clinical trials have been conducted to test its effectiveness in anticancer therapy. Angiostatin is a fragment of a larger protein called plasmin and it contains three to five kringle modules. There are four different variations of angiostatin, but studies have shown that the K1-3 variation is the most important for its inhibitory effect. The structure of angiostatin is divided into two sides, with one side responsible for inhibiting cellular proliferation and the other side responsible for inhibiting cell migration. Angiostatin can be produced by cutting plasminogen or plasmin (a protein in the blood that helps dissolve blood clots) into smaller pieces. This cutting process is done by different enzymes called metalloproteinases, elastase, prostate-specific antigen (PSA), 13 KD serine protease, or 24KD endopeptidase. Angiostatin is known to bind to various proteins, including angiomotin, ATP synthase, integrins, and proteoglycans, but its exact mechanism of action is still uncertain. It is believed to inhibit endothelial cell migration and proliferation and induce apoptosis. Three proposed mechanisms of action include inhibition of ATP production and regulation of pH in tumor cells, reduction of epithelial cell migration by binding to integrins, and activation of focal adhesion kinase through binding to angiomotin. Further research is needed to fully understand how angiostatin works.

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